Biological treatment of wastewater for removal of dissolved organics is well known and is widely practiced in both municipal and industrial plants. This aerobic biological process is generally known as the “activated sludge” process in which micro-organisms consume the organic compounds through their growth. The process necessarily includes sedimentation of the micro-organisms or “biomass” to separate it from the water and complete the process of reducing Biological Oxygen Demand (BOD) and TSS (Total Suspended Solids) in the final effluent. The sedimentation step is typically done in a clarifier unit. Thus, the biological process is constrained by the need to produce biomass that has good settling properties. These conditions are especially difficult to maintain during intermittent periods of high organic loading and the appearance of contaminants that are toxic to the biomass.
Typically, this activated sludge treatment has a conversion ratio of organic materials to sludge of about 0.5 kg sludge/kg COD (chemical oxygen demand), thereby resulting in the generation of a considerable amount of excess sludge that must to be disposed of. The expense for the excess sludge treatment has been estimated at 40-60 percent of the total expense of wastewater treatment plant. Moreover, the conventional disposal method of landfilling may cause secondary pollution problems. Therefore, interest in methods to reduce the volume and mass of the excess sludge has been growing rapidly.
Membranes coupled with biological reactors for the treatment of wastewater are well known, but are not widely practiced. In these systems, ultrafiltration (UF), microfiltration (MF) or nanofiltration (NF) membranes replace sedimentation of biomass for solids-liquid separation. The membrane can be installed in the bioreactor tank or in an adjacent tank where the mixed liquor is continuously pumped from the bioreactor tank and back producing effluent with much lower total suspended solids (TSS), typically less than 5 mg/L, compared to 20 to 50 mg/L from a clarifier.
More importantly, MBRs (membrane biological reactors) de-couple the biological process from the need to settle the biomass, since the membrane sieves the biomass from the water. This allows operation of the biological process at conditions that would be untenable in a conventional system including: 1) high MLSS (bacteria loading) of 10-30 g/L, 2) extended sludge retention time, and 3) short hydraulic retention time. In a conventional system, such conditions could lead to sludge bulking and poor settleability.
The benefits of the MBR operation include low sludge production, complete solids removal from the effluent, effluent disinfection, combined COD, solids and nutrient removal in a single unit, high loading rate capability, no problems with sludge bulking, and small footprint. Disadvantages include aeration limitations, membrane fouling, and membrane costs.
Membrane costs are directly related to the membrane area needed for a given volumetric flow through the membrane, or “flux.” Flux is expressed as liters/hour/m2 (LMH) or gallons/day/ft2 (GFD). Typical flux rates vary from approximately 10 LMH to about 50 LMH. These relatively low flux rates, due largely to fouling of the membranes, have slowed the growth of MBR systems for wastewater treatment.
The MBR membrane interfaces with so-called “mixed liquor” which is composed of water, dissolved solids such as proteins, polysaccharides, suspended solids such as colloidal and particulate material, aggregates of bacteria or “flocs”, free bacteria, protozoa, and various dissolved metabolites and cell components. In operation, the colloidal and particulate solids and dissolved organics deposit on the surface of the membrane. Colloidal particles form layer on the surface of the membrane called a “cake layer.” Cake layer formation is especially problematic in MBRs operated in the “dead end” mode where there is no cross flow; i.e., flow tangential to the membrane. Depending on the porosity of the cake layer, hydraulic resistance increases and flux declines.
In addition to the cake formation on the membrane, small particles can plug the membrane pores, a fouling condition that may not be reversible. Compared to a conventional activated sludge process, floc (particle) size is reportedly much smaller in typical MBR units. Since MBR membrane pore size varies from about 0.04 to about 0.4 micrometers, particles smaller than this can cause pore plugging. Pore plugging increases resistance and decreases flux.
Therefore, there is an ongoing need to develop improved methods of conditioning the mixed liquor in MBR units to increase flux and reduce fouling of the membranes.